If it has a sticker on it (the desktop, I mean) then the XP product key on it is likely an OEM key. As to whether this would work witha non-OEM disk, I'm not sure. Maybe.
Try contacting the manufacturer of the desktop, pretend to be the original owner (just for simplicity's sake) and asking for the restore CD for that model. If they have one and are willing to give it to you, you may only need to pay for postage (but they may also charge a fee).
Installing XP is pretty easy. Here's the MS article about installing XP: http://support.microsoft.com/kb/316941
plus you can always ask us for additional help! Go ahead, we like it!